As stated the Ambient, meaning "surrounding air" temp can be anything from the room air temp to the temp surrounding the CPU. When my free standing a/c was running the supply air temps effected the perceived ambient causing me to have to time the a/c units cycles before taking a temp reading. I was not saying this testing is all wasted just that since we can not control the variables precisely there can be a large room for error. A torture rack would make for a better test bed since there are no enclosures to trap air.
well I am a little confused about how to do an accurate calibration. If it's an air sink case closed you would calibrate to the case ambient not the room Ambient temp. 10 C difference in the calibration number?

Water would seem more complicated, I have not had a chance to noodle it through completely but. Open case cpu would be some few degrees higher than the ambient room, higher than the motherboard temp so with a small delta you probably have some heat flow to the MB? marginal at best. Now case closed Ambient would be higher in the case but the MB Temp is now higher than the CPU because you have an exterior radiator to ambient room. Deltas have reversed so now you are drawing heat from the motherboard. So how large a delta? The larger the more influence on the calibration.